A storage medium is part of a computer's memory wherein programs and work files reside as digital data. Computer memory can include either, and commonly includes both, moving-type memory and non-moving type memory. Nonmoving memory is typically directly addressed, or accessed, by the computer's central processing unit. Moving memory, such as disk drives and magnetic tape, is indirectly accessed.
Moving memory typically has much greater data storage capacity than directly addressed memory and has much longer access times. Moving memory is also typically not volatile. That is, it survives turning the computer off. Nonmoving type memory is typically faster and more expensive per unit of memory than moving-type memory, and has less capacity. Moving-type memories are generally used for long-term storage of large programs and substantial bodies of information, such as a data base files, which are not in constant use by the computer, or which are too bulky to provide short term direct access memory capacity for.
The storage media of the moving-type memory are physically alterable objects. That is to say, they can be magnetized, grooved, pitted or altered in some detectible fashion to record information. Preferably the storage media is at the same time physically resilient, portable, cheap, of large capacity, and resistant to accidental alteration. A crude example of an analogous medium is a phonograph record wherein a wavy spiral groove represents an analog information signal. The various species of storage media used in moving-type memory for computers include magnetic tape, floppy disks, compact disk-ROM, Write-Once, Read-Many optical disks and, most recently, erasable magneto-optic disks. Each of these storage media exhibit detectable physical changes to the media representing binary data. To read, and where applicable to erase and write data to the media, mechanical apparatuses are provided which can be directed to the proper location on the physical media and carry out the desired function.
Magneto-optic storage disks are similar in appearance to optical compact disks used for storing recorded music. In one common form the disks are five and a quarter inch diameter flat disks having a central axis for being engaged and spun by a drive motor. The disk encloses a material of known reflective properties encased in a hard, transparent protective shell. In erasable magneto-optic disks the reflective material is also a magnetic material which can support local magnetic domains or regions. The reflection characteristics of the surface of the disk depend upon the local magnetic domain state. Detection of the changes in reflectivity is enhanced by polarizing the incident radiation hitting the surface of the optical disk. While the changes in reflection characteristics are subtle, the orientation of the magnetic dipole in each local region can be detected by measuring selected phase characteristics of the polarized light reflected by the region. Thus each local region can represent one piece of binary data with the orientation of the magnetic dipole being associated with the data value.
The magnetic field of the material is reversible and thus erasure and repeated rewriting on the disk is possible. When the reflective material is in its solid phase, the magnetic domains are substantially locked. When the material is in its liquid phase, an outside magnetic field can be used to set the magnetization of the material, which is substantially locked upon solidification of the material. Where the liquid phase is limited to a local region, the magnetic polarization of just that local region can be changed.
Liquification of a local region is provided by heating the selected local region with a laser beam and applying a magnetic field to the region during the resulting liquid phase. The area freezes while the field is present, thus setting the magnetic polarization of the local region. A focusing or objective lens is used to focus the laser beam at the surface of the disk. The tighter the focus of the laser beam used to heat the region, the smaller this region will be, reducing the power requirements to melt the material. Regions can then be allowed closer to one another increasing data density.
The local regions in which data are stored are typically arranged serially in a plurality of concentric tracks on the face of a disk. Groups of local regions are identified by track and sector designation relating to a coordinate system for locating and relocating the local regions. The tracks may be a portion of a spiral groove on the face of the disk, similar to the groove in a record, or they can be a series of concentric grooves.
The grooves are optically detectable allowing a read/write head to be oriented over the center of the groove while the disk spins underneath the head. The read/write head carries a laser source, the objective lens for focusing the laser beam and an optical detector for developing positioning signals for the objective lens. One of the positioning signals is generated by a tracking servo loop, which operates to center the read/write head, and thereby the objective lens, over the groove. Another positioning signal is generated by a focus signal servo loop and brings a focal point of the objective lens to the surface of the erasable disk.
The same laser source is used for a read operation, a write operation and an erase operation. Distinct power levels exist for each of these operations. Particularly during write and erase operations, close maintenance of the focal point at the disk surface is necessary to bring sufficient energy to a local region area to melt the region. Because the surface of the disk is not perfectly flat, the focal point must be constantly moved to maintain focus of the beam at the surface of the disk. Movement of the focal point is done by moving the objective lens.
The optical detector includes an array of optical sensors located behind the objective lens which produce output signals in response to laser light reflected by the surface of the disk impinging on the sensors. The radial distribution of energy around the central axis of the laser beam is functionally related to the output signals from the sensors. The output signals can thus be related to the position of the focal point. During the read operation the distribution of power in the beam is substantially symmetric about any axis cutting perpendicularly through the center of the beam. A signal processor can readily operate on the outputs of the optical detector to properly position the focal point. During the erase operation this is not the case. The energy distribution of the laser beam can become strongly asymmetric in patterns not predictable from laser to laser. This phenomenon is known as beam farfield shift. The asymmetric energy distribution of the reflected laser light can result in false indications of loss of focus and in false indications of proper focus when in fact optimal focus has been lost.
Some prior art magneto-optic memory systems have ignored this problem, which can lead to the failure to completely erase a sector of memory regions. Other prior art devices have locked the position of the objective lens in the position generated in the most recent read operation. Unfortunately, magneto-optic disk surfaces are not perfectly flat, and this approach results in a loss of focus stemming from movement of the disk. Again incomplete erasure can result.